The optical properties of nonlinear optical (NLO) materials are modified by light passing through the materials. The modification of the optical properties may be caused by an induced electronic charge displacement (polarization) that acts as an oscillating dipole. The oscillating dipole may cause the material to emit a photon. When the polarization of the material is linear, the emitted photon has the same frequency as the light incident upon the material. If the polarization is nonlinear, the frequency of the light emerging from the material may be some integer value times the frequency of the incident light. For example, the net effect of frequency doubling is that two photons with a frequency Ω combine to generate a single photon having a frequency equal to 2Ω. Thus, propagation of the waves in synchronization (phase-matching) allows the light's frequency to double. Frequency doubling also is referred to as second harmonic generation (SHG). NLO materials were discovered to be capable of second harmonic generation in 1961. Ann. Rev. Mater. Sci., 16:203-43 (1986).
Several NLO materials have been recognized in the prior art. For example, YAl3(BO3)4 NLO crystals are disclosed in Pu Wang ET AL., Growth And Evaluation Of Ytterbium-Doped Yttrium Aluminum Borate as a Potential Self-Doubling Laser Crystal, J. OF THE OPTICAL SOC. OF AM. B, January 1999, at 63-69, which is incorporated herein by reference. NLO crystals, such as YAl3(BO3)4 NLO crystals, have been used in a variety of devices. For example, U.S. Pat. No. 5,202,891 describes a neodymium yttrium aluminum borate NLO crystal incorporated into a laser device. U.S. Pat. No. 4,826,283 describes another NLO device made from single crystals of LiB3O5.
It is known to recrystallize NLO materials to form larger crystals with uniform crystal structures. For example, J. Li ET AL. disclose the formation of YAl3(BO3)4 NLO crystals from a flux containing K2MoO2. See J. Li ET AL., The Influence of Yb3+ Concentration on Yb:YAl3(BO3)4, 38 CRYS. RES. TECH. 890-895 (2003), which is incorporated herein by reference. Aluminum-borate NLO crystals grown with conventional techniques are generally limited to the generation or emission of light at wavelengths greater than 370 nm.